The contraction of the brow occurs in other cases besides those mentioned. Before all it occurs when anything is dealt with intensively, increasing with the increase of the difficulty of the subject. The aboriginal source of this gesture lies in the fact that intensive activities involve the need of acuter vision, and this is in some degree acquired by the contraction of the skin of the forehead above the eyebrows; for vision is clarified in this way. Intensive consideration on the part of a defendant or a witness, and the establishment of its reality or simulation, are significant in determining whether he himself believes the truth of what is about to be explained. Let us suppose that the issue involves proving an alibi on a certain definite, rather remote day, and the defendant is required to think over his whereabouts on that day. If he is in earnest with regard to the establishment of his alibi, i.e. if he really was not there and did not do the thing, it will be important for him to remember the day in question and to be able to name the witnesses of his whereabouts then. Hence he will think intensively. But if he has claimed an alibi dishonestly, as is frequent with criminals, in order to make people conclude that nobody has the right to demand where and for how long a time he was on such and such a day, then there is no need of thinking closely about something that has not happened. He exhibits in such cases a kind of thoughtfulness, which is not, however, earnest and profound: and these two adjectives describe _real_ consideration. The same observations are to be made in regard to dishonest witnesses who, when pressed to think hard, only simulate doing so. One is compelled at the very least to look closely after the witness who simply imitates intensive thinking without showing the signs proper to it. The suspicion of false testimony is then justifiable.